Are you suggesting that they "smell" the human blood and are deterred? I mean, that would make sense if human remains haven't been found in the stomachs of white sharks before. The thing is...they have.
Typically, a white shark doesn't smell the blood of its prey before the attack. They want their presence to be completely unknown until the moment they strike. That's when you see the ambush strategy employed: one massive hit, and ask questions later.
However, there have been cases in which people reported feeling something scrape or rub against a leg or foot before an attack. Sharks have tiny teeth (denticles) on their skin that can be highly abrasive. Interestingly, there are small pores/organs underneath these denticles that appear similar to taste buds. The idea has in the past been put forth that sharks use their skin-teeth (denticles) to create abrasions on would-be prey items in order to "taste" them.
Furthermore, when exploring unfamiliar objects (perhaps testing them for edibility), sharks use their mouths in place of hands. It wouldn't be surprising to hear that a shark, before a potential attack, has gently "bitten" a victim to test it out. That's why you often hear about shark attack survivors feeling an initial "tug" or other "gentle" sensation on their leg or foot before an actual attack took place.
So, I suppose in summary, sometimes sharks do take a cautious approach to unfamiliar and potential prey items. However, I wouldn't characterize most attacks by white sharks in that way based on accounts of the attacks. Those are usually consistent with the ambush-strategy attack. The intent is to kill, not to explore.
I know I'm rambling, but I just wasn't sure how to approach your question, or what it was you were getting at.
